This invention relates generally to combustors for gas turbines and more particularly concerns a combustor using lean direct injection for reduced NO.sub.x emissions.
Traditional gas turbine combustors use nonpremixed ("diffusion") flames in which fuel and air freely enter the combustion chamber separately. Typical diffusion flames are dominated by regions which burn at or near stoichiometric conditions. The resulting flame temperatures can exceed 3900.degree. F. Because diatomic nitrogen rapidly disassociates at temperatures exceeding about 3000.degree. F., diffusion flames typically produce unacceptably high levels of NO.sub.x emissions. One method commonly used to reduce peak temperatures (and thereby reduce NO.sub.x emissions) is to inject water or steam into the combustor, but this technique is expensive in terms of process steam or water and can have the undesirable side effect of quenching CO burnout reactions.
Lean premixed injection is a potentially more attractive approach to lowering peak flame temperature than water or steam injection. In lean premixed combustion, fuel and air are premixed in a premixer section, and the fuel-air mixture is injected into a combustion chamber where it is burned. Due to the lean stoichiometry resulting from the premixing, lower flame temperatures, and therefore lower NO.sub.x emissions, are achieved. However, the fuel-air mixture is generally flammable, and undesirable flashback into the premixer section is possible. Furthermore, gas turbine combustors utilizing lean premixed combustion typically require some conversion from premixed to diffusion operation at turndown conditions to maintain a stable flame. Such conversion capability introduces design complexities and generally raises costs.
Accordingly, there is a need for a dry low NO.sub.x combustion system which does not require premixing of fuel and air prior to combustion.